The present invention relates to the general field of thrusters or rocket engines for delivering thrust for driving vehicles such as missiles, launchers, or indeed satellites, by using the principle of propulsion by reaction or by ejecting gas. The invention relates more precisely, but not exclusively, to the nozzles that are fitted to solid propellant thrusters.
A solid propellant thruster is constituted mainly by a casing enclosing a block of propellant, an igniter, and a non-variable diverging nozzle. The block of propellant is pierced by a channel situated on the axis of the thruster, which channel serves as a combustion chamber. The igniter serves to ignite the propellant at one end of the casing, and combustion of the propellant propagates from the front towards the rear of the thruster. The propellant burns at a predefined speed, thereby producing combustion gas that is expelled via the nozzle.
The throat section of the nozzles serves to regulate combustion of the block of propellant in such a manner as to maintain the desired pressure inside the combustion chamber, while producing the expected thrust. Also, for a thruster that delivers thrust at a single rate, the throat section of the nozzle is not variable and is predetermined as a function of the desired level of thrust.
Nevertheless, having recourse to a single throat section is not appropriate for a thruster having two operating rates, typically one when operating at a high rate (acceleration) and another when operating at a lower rate (cruising), or for a thruster that is to fly at varying altitude.
In order to mitigate such a drawback, one known solution consists in providing the launcher with two distinct thrusters: one for an acceleration or “boost” stage, and another for a cruising stage. Nevertheless, such a solution has the drawback of significantly increasing the cost of the launcher.
It is also known to provide a nozzle with a throat of section that is variable. In practice, a plunger that is movable in translation is housed inside the nozzle. The position of the plunger in the flow of combustion gas serves to determine the flow section for the gas leaving the nozzle, and thus to adjust the ejection section of the gas to the operating rate of the thruster.
Nevertheless, although the use of a plunger provides a solution that is inexpensive and that enables a plurality of operating rates to be obtained in a single nozzle, thrust efficiencies are not optimized for those various operating rates.